Movatterモバイル変換

Network Working Group                                             L. ZhuRequest for Comments: 4537                                      P. LeachUpdates:4120                                              K. JaganathanCategory: Standards Track                          Microsoft Corporation                                                               June 2006Kerberos Cryptosystem Negotiation ExtensionStatus of This Memo   This document specifies an Internet standards track protocol for the   Internet community, and requests discussion and suggestions for   improvements.  Please refer to the current edition of the "Internet   Official Protocol Standards" (STD 1) for the standardization state   and status of this protocol.  Distribution of this memo is unlimited.Copyright Notice   Copyright (C) The Internet Society (2006).Abstract   This document specifies an extension to the Kerberos protocol as   defined inRFC 4120, in which the client can send a list of supported   encryption types in decreasing preference order, and the server then   selects an encryption type that is supported by both the client and   the server.Table of Contents1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . . .22.  Conventions Used in This Document . . . . . . . . . . . . . . .23.  Negotiation Extension . . . . . . . . . . . . . . . . . . . . .24.  Security Considerations . . . . . . . . . . . . . . . . . . . .45.  Acknowledgements  . . . . . . . . . . . . . . . . . . . . . . .46.  Normative References  . . . . . . . . . . . . . . . . . . . . .4Zhu, et al.                 Standards Track                     [Page 1]

RFC 4537                  Enctype Negotiation                  June 20061.  Introduction   Under the current mechanism [RFC4120], the Kerberos Distribution   Center (KDC) must limit the ticket session key encryption type   (enctype) chosen for a given server to one it believes is supported   by both the client and the server.  If both the client and server   understand a stronger enctype than the one selected by the KDC, they   cannot negotiate it.  As the result, the protection of application   traffic is often weaker than necessary when the server can support   different sets of enctypes depending on the server application   software being used.   This document specifies an extension to the Kerberos protocol to   allow clients and servers to negotiate use of a different and   possibly stronger cryptosystem in subsequent communication.   This extension utilizes an authorization data element in the   authenticator of the AP-REQ message [RFC4120].  The client sends the   list of enctypes that it supports to the server; the server then   informs the client of its choice.  The negotiated subkey is sent in   the AP-REP message [RFC4120].2.  Conventions Used in This Document   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",   "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this   document are to be interpreted as described in [RFC2119].3.  Negotiation Extension   If the client prefers an enctype over that of the service ticket   session key, then it SHOULD send a list of enctypes in decreasing   preference order to the server.  Based on local policy, the client   selects enctypes out of all the enctypes available locally to be   included in this list, and it SHOULD NOT include enctypes that are   less preferable than that of the ticket session key in the service   ticket.  In addition, the client SHOULD NOT include negative (local-   use) enctype numbers unless it knows a priori that the server has   been configured to use the same negative enctype numbers for the same   enctypes.   The client sends the enctype list via the authorization-data of the   authenticator in the AP-REQ [RFC4120].  A new authorization data   element type AD-ETYPE-NEGOTIATION is defined.           AD-ETYPE-NEGOTIATION              129Zhu, et al.                 Standards Track                     [Page 2]

RFC 4537                  Enctype Negotiation                  June 2006   This authorization data element itself is enclosed in the AD-IF-   RELEVANT container; thus, a correctly implemented server that does   not understand this element should ignore it [RFC4120].  The value of   this authorization element contains the DER [X680] [X690] encoding of   the following ASN.1 type:           EtypeList ::= SEQUENCE OF Int32              -- Specifies the enctypes supported by the client.              -- This enctype list is in decreasing preference order              -- (favorite choice first).              -- Int32 is defined in [RFC4120].   If the EtypeList is present and the server prefers an enctype from   the client's enctype list over that of the AP-REQ authenticator   subkey (if that is present) or the service ticket session key, the   server MUST create a subkey using that enctype.  This negotiated   subkey is sent in the subkey field of AP-REP message, and it is then   used as the protocol key or base key [RFC3961] for subsequent   communication.   If the enctype of the ticket session key is included in the enctype   list sent by the client, it SHOULD be the last on the list;   otherwise, this enctype MUST NOT be negotiated if it was not included   in the list.   This negotiation extension SHOULD NOT be used when the client does   not expect the subkey in the AP-REP message from the server.   A note on key generation: The KDC has a strong Pseudo-Random Number   Generator (PRNG); as such, the client can take advantage of the   randomness provided by the KDC by reusing the KDC key data when   generating keys.  Implementations SHOULD use the service ticket   session key value as a source of additional entropy when generating   the negotiated subkey.  If the AP-REQ authenticator subkey is   present, it MAY also be used as a source of entropy.   The server MAY ignore the preference order indicated by the client.   The policy by which the client or the server chooses an enctype   (i.e., how the preference order for the supported enctypes is   selected) is a local matter.Zhu, et al.                 Standards Track                     [Page 3]

RFC 4537                  Enctype Negotiation                  June 20064.  Security Considerations   The client's enctype list and the server's reply enctype are part of   encrypted data; thus, the security considerations are the same as   those of the Kerberos encrypted data.   Both the EtypeList and the server's sub-session key are protected by   the session key or sub-session key used for the AP-REQ, and as a   result, if a key for a stronger enctype is negotiated underneath a   key for a weaker enctype, an attacker capable of breaking the weaker   enctype can also discover the key for the stronger enctype.  The   advantage of this extension is to minimize the amount of cipher text   encrypted under a weak enctype to which an attacker has access.5.  Acknowledgements   The authors would like to thank the following individuals for their   comments and suggestions: Ken Raeburn, Luke Howard, Tom Yu, Love   Hornquist Astrand, Sam Hartman, and Martin Rex.6.  Normative References   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate              Requirement Levels",BCP 14,RFC 2119, March 1997.   [RFC3961]  Raeburn, K., "Encryption and Checksum Specifications for              Kerberos 5",RFC 3961, February 2005.   [RFC4120]  Neuman, C., Yu, T., Hartman, S., and K. Raeburn, "The              Kerberos Network Authentication Service (V5)",RFC 4120,              July 2005.   [X680]     ITU-T Recommendation X.680 (2002) | ISO/IEC 8824-1:2002,              Information technology - Abstract Syntax Notation One              (ASN.1): Specification of basic notation.   [X690]     ITU-T Recommendation X.690 (2002) | ISO/IEC 8825-1:2002,              Information technology - ASN.1 encoding Rules:              Specification of Basic Encoding Rules (BER), Canonical              Encoding Rules (CER) and Distinguished Encoding Rules              (DER).Zhu, et al.                 Standards Track                     [Page 4]

RFC 4537                  Enctype Negotiation                  June 2006Authors' Addresses   Larry Zhu   Microsoft Corporation   One Microsoft Way   Redmond, WA  98052   US   EMail: lzhu@microsoft.com   Paul Leach   Microsoft Corporation   One Microsoft Way   Redmond, WA  98052   US   EMail: paulle@microsoft.com   Karthik Jaganathan   Microsoft Corporation   One Microsoft Way   Redmond, WA  98052   US   EMail: karthikj@microsoft.comZhu, et al.                 Standards Track                     [Page 5]

RFC 4537                  Enctype Negotiation                  June 2006Full Copyright Statement   Copyright (C) The Internet Society (2006).   This document is subject to the rights, licenses and restrictions   contained inBCP 78, and except as set forth therein, the authors   retain all their rights.   This document and the information contained herein are provided on an   "AS IS" basis and THE CONTRIBUTOR, THE ORGANIZATION HE/SHE REPRESENTS   OR IS SPONSORED BY (IF ANY), THE INTERNET SOCIETY AND THE INTERNET   ENGINEERING TASK FORCE DISCLAIM ALL WARRANTIES, EXPRESS OR IMPLIED,   INCLUDING BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF THE   INFORMATION HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED   WARRANTIES OF MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE.Intellectual Property   The IETF takes no position regarding the validity or scope of any   Intellectual Property Rights or other rights that might be claimed to   pertain to the implementation or use of the technology described in   this document or the extent to which any license under such rights   might or might not be available; nor does it represent that it has   made any independent effort to identify any such rights.  Information   on the procedures with respect to rights in RFC documents can be   found inBCP 78 andBCP 79.   Copies of IPR disclosures made to the IETF Secretariat and any   assurances of licenses to be made available, or the result of an   attempt made to obtain a general license or permission for the use of   such proprietary rights by implementers or users of this   specification can be obtained from the IETF on-line IPR repository athttp://www.ietf.org/ipr.   The IETF invites any interested party to bring to its attention any   copyrights, patents or patent applications, or other proprietary   rights that may cover technology that may be required to implement   this standard.  Please address the information to the IETF at   ietf-ipr@ietf.org.Acknowledgement   Funding for the RFC Editor function is provided by the IETF   Administrative Support Activity (IASA).Zhu, et al.                 Standards Track                     [Page 6]